NATURAL PHILOSOPHY. 165 



ON THE CONSTRUCTION OF A GALVANOMETER FOR THE DETEC- 

 TION OF WEAK ELECTRIC CURRENTS. 



At tlie 1868 meeting of the British Association, Mr. F. II. Varley 

 explained that the smaller the magnet used in a galvanometer 

 was, the greater would lie the sensitiveness of the instrument. 

 The small magnet of Sir W. Thomson liad a mirror attached to it 

 to reliect a beam of light, so that a small motion of the magnet 

 gives movement to this imponderable indicating arm, and which 

 is rendered apparent by the movement of a line of light upon a 

 darkened scale. But it had frequently occurred to the author that 

 smaller and lighter magnets might be employed by calling in the 

 aid of microscopic power, and the instrnment which he had con- 

 structed took two forms, both of which had been found to answer 

 exceedingly well. The first consists in suspending, with a single 

 filament of silk, a magnet made of the finest steel wire that can 

 be obtained, and rendering its motion apparent by viewin2" it 

 through a rectangular prism by means of a microscope, in the 

 eye-piece of which is placed a small graduated scale, photographed 

 on glass. The magnet appears to be a black bar bisecting the 

 field of view ; and as the finest wire obtainable for this purpose 

 appears as thick as a scaffold-pole when sufficiently magnified, it 

 is obvious that the slightest motion of the mas^netmust be rendered 

 conspicuous by the image moving to and fro over the graduated 

 scale placed in the eye-piece. The second form is more sensitive 

 than the first. A small magnet, made of flat steel, polished on one 

 face, is suspended in the usual manner by a single filament of 

 silk, and a small mico-photograph of a graduated scale is placed 

 at such a distance from the reflecting surface that each of the 

 photographed divisions shall equal two minutes of arc as nearly 

 as possible. The image of the scale thus reflected is sent in a 

 line with the optic axis of the microscope, and any deflection 

 given to the magnet causes the piiotographed scale to appear to 

 move across the field of view. The reflecting surface moving 

 doubles the apparent motion, giving the amount due to the angle 

 of incidence, plus reflection. A movement of one graduated 

 division being produced by one minute of deflection, if magnified 

 60 times by the microscope, will render a motion equal to 1 sec- 

 ond of arc apparent. Where desirable, a small scale placed in the 

 eye-piece can be made to give a vernier reading upon the mag- 

 nified scale. The magnifying power can be increased where 

 desired, and very minute amounts of motion rendered measurable. 

 The great difficulty of using instruments of such extreme sensibi- 

 bility, wdfich are interfered with by extraneous vibrations, can be 

 to a great extent overcome, by insulating the various parts from 

 vibration by means of antagonizing spiral springs, and preventing 

 finer vibrations from l)eing communicated throush the wire itself 

 by covering the wire with silk or cotton wrappings, to act as damp- 

 ers to the wire. The advantage of this galvanometer consisted 

 in the fact, that it was not necessary that the place where it was 



